linux-kernel - Re: [PATCH] powerpc/irq: Run softirqs off the top of the irq stack

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Date:	Mon, 23 Sep 2013 17:56:21 +1000
From:	Stephen Rothwell <sfr@...b.auug.org.au>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
	James Hogan <james.hogan@...tec.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"David S. Miller" <davem@...emloft.net>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Helge Deller <deller@....de>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	"James E.J. Bottomley" <jejb@...isc-linux.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Paul Mackerras <paulus@....ibm.com>
Subject: Re: [PATCH] powerpc/irq: Run softirqs off the top of the irq stack

Hi Ben,

On Mon, 23 Sep 2013 14:35:58 +1000 Benjamin Herrenschmidt <benh@...nel.crashing.org> wrote:
>
> diff --git a/arch/powerpc/kernel/irq.c b/arch/powerpc/kernel/irq.c
> index c69440c..0c9646f 100644
> --- a/arch/powerpc/kernel/irq.c
> +++ b/arch/powerpc/kernel/irq.c
> @@ -443,46 +443,7 @@ void migrate_irqs(void)
>  
>  static inline void handle_one_irq(unsigned int irq)
>  {
> -	struct thread_info *curtp, *irqtp;
> -	unsigned long saved_sp_limit;
> -	struct irq_desc *desc;
> -
> -	desc = irq_to_desc(irq);
> -	if (!desc)
> -		return;
> -
> -	/* Switch to the irq stack to handle this */
> -	curtp = current_thread_info();
> -	irqtp = hardirq_ctx[smp_processor_id()];
> -
> -	if (curtp == irqtp) {
> -		/* We're already on the irq stack, just handle it */
> -		desc->handle_irq(irq, desc);
> -		return;
> -	}
> -
> -	saved_sp_limit = current->thread.ksp_limit;
> -
> -	irqtp->task = curtp->task;
> -	irqtp->flags = 0;
> -
> -	/* Copy the softirq bits in preempt_count so that the
> -	 * softirq checks work in the hardirq context. */
> -	irqtp->preempt_count = (irqtp->preempt_count & ~SOFTIRQ_MASK) |
> -			       (curtp->preempt_count & SOFTIRQ_MASK);
>  
> -	current->thread.ksp_limit = (unsigned long)irqtp +
> -		_ALIGN_UP(sizeof(struct thread_info), 16);
> -
> -	call_handle_irq(irq, desc, irqtp, desc->handle_irq);
> -	current->thread.ksp_limit = saved_sp_limit;
> -	irqtp->task = NULL;
> -
> -	/* Set any flag that may have been set on the
> -	 * alternate stack
> -	 */
> -	if (irqtp->flags)
> -		set_bits(irqtp->flags, &curtp->flags);
>  }

This function ends up as a single blank line ...

> @@ -519,18 +480,64 @@ void do_IRQ(struct pt_regs *regs)
>  	 */
>  	irq = ppc_md.get_irq();
>  
> -	/* We can hard enable interrupts now */
> +	/* We can hard enable interrupts now to allow perf interrupts */
>  	may_hard_irq_enable();
>  
>  	/* And finally process it */
> -	if (irq != NO_IRQ)
> -		handle_one_irq(irq);

then you remove the only call, so why not just remove the function
completely?

-- 
Cheers,
Stephen Rothwell                    sfr@...b.auug.org.au

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